1. Field of the Invention
The present invention relates to a head-retract circuit for a moving media data storage apparatus, and more particularly to a head-retract circuit powered by the inductive flyback voltage spikes and back emf voltage generated by a motor of the moving media storage apparatus.
2. Description of the Prior Art
Moving media data storage devices, such as hard disk drives and optical disk drives, are typically incorporated into battery-powered portable computers to provide non-volatile memory storage. A disk drive includes a spindle motor for rotating one or more information-bearing disks and a voice-coil motor for positioning one or more read/write heads adjacent desired sectors of the information-bearing disks. When the spindle motor is stopped, the read/write heads come to rest on safe sectors of the disks; that is, sectors of the disks which do not contain encoded data. The safe sectors are typically located at the inner or outer periphery of the disk. The read/write heads are aerodynamically formed such that they fly over the data bearing surface on air currents created by the rotation of the disk.
Unexpected disconnection or failure of the battery can result in a loss of memory and, in some cases, damage to the moving media data storage device. During normal power-down procedures, the read/write heads are retracted to the safe sectors before they contact the surface of the disks. However, when an unexpected loss of battery power occurs and the air currents supporting the read/write heads are lost, the read/write heads can "crash" against sectors of the disks which contain encoded data, thereby causing a loss of stored information and possibly damaging the disks.
To prevent head damage or loss of stored information, the head-retract circuitry is typically powered by the back electromotive force (emf) generated by the rotating motor when the battery is suddenly lost. During the emergency head-retract procedure, the motor, acting as a generator, supplies power to the voice-coil motor such that the read/write head is moved to a safe sector of the disk before it comes into contact with the disk.
FIG. 1 shows a simplified circuit diagram of a moving media storage device 800 which includes a prior art head actuator 810 having circuitry for automatically retracting a read/write head (not shown). The storage device 800 also includes a spindle driver 820 for driving a spindle motor 830, a voice-coil motor 840 connected to the head actuator 810, and a Schottky diode 850. A battery 805, which is typically located outside of the storage device 800, is connected to an anode of the Schottky diode 850 to provide an operating potential. In this description of the prior art, the voltage provided by the battery 805 is referred to as V.sub.BAT, and the operating potential applied to the spindle driver 820 and the head actuator 810 (after a voltage drop associated with the Schottky diode 850) is referred to as V.sub.CC.
The spindle driver 820 is connected between V.sub.CC and ground, and generates a driving signal which is transmitted to a spindle motor 830. The spindle driver 820 includes a servo controller 821, for generating control signals, and three push-pull halfbridge circuits 822, 823 and 824, each comprising a highside MOSFET 825 and a lowside MOSFET 826.
The sources and bodies of MOSFETs 825 and 826 are shorted together. Thus each of the highside MOSFETs 825 includes an intrinsic antiparallel diode A1 between its drain and source which is reverse-biased during normal current flow through MOSFET 825. Similarly, each of the lowside MOSFETs 826 includes an intrinsic antiparallel diode A2 between its drain and source which is reverse-biased during normal current flow. Halfbridges 822, 823 and 824 produce output signals V.sub.OUTA, V.sub.OUTB and V.sub.OUTC, which are delivered to respective input terminals of motor 830. The head actuator 810 is connected between V.sub.CC and ground, and generates a head positioning signal which is transmitted to voice-coil motor 840. The head actuator 810 includes a head-control circuit 811, a head-retract circuit 812 and two push-pull halfbridge circuits 813 and 814, each comprising a highside MOSFET 815 and a lowside MOSFET 816, which are connected together source-to-drain in a totem pole manner. The source and body of each of MOSFETs 815 and 816 are shorted. The head control circuit 811 receives a sector identification signal from a host computer (not shown) and feedback signals from a read/write head, and generates control signals which are applied to the gates of the highside MOSFETs 815 and lowside MOSFETs 816. The head-retract circuit 812 detects a power failure and applies head-retract control signals to the gates of the highside MOSFETs 815 and the lowside MOSFETs 816. Halfbridges 813 and 814 generate output signals V.sub.OUTD and V.sub.OUTE, which are delivered to respective terminals of voice-coil motor 840. Both the highside MOSFETs 815 and the lowside MOSFETs 816 include intrinsic antiparallel diodes (not numbered) between their sources and drains. The gate of each of MOSFETs 815 and 816 is connected to receive control signals generated by the head-controller 811, such that the halfbridges generate a head positioning signal (V.sub.OUTD and V.sub.OUTE) to drive the voice-coil motor 840 so as to move the read/write head over a safe sector of the disk.
In operation, V.sub.BAT is applied to the anode of the Schottky diode 850, thereby producing V.sub.CC which is approximately 0.5 volts lower than V.sub.BAT. V.sub.CC is applied to the spindle driver 820 and the head actuator 810.
In the spindle driver, in a known manner the servo control circuit 821 generates control signals which are applied to the highside MOSFETs 825 and lowside MOSFETs 826 of the halfbridges 822, 823 and 824. For example, a high control signal applied to the gate of MOSFET 825 causes the MOSFET to turn on and to apply V.sub.CC to one pole of the spindle motor 830. At the same time, a low control signal is applied to the gate of MOSFET 826 thereby connecting the other pole of motor 830 to ground and creating a driving potential in the spindle motor 830. The servo control circuit 821 alternates the control signals applied to the highside MOSFETs 825 and the lowside MOSFETs 826 to produce a three-phase driving signal which causes the spindle motor 830 to rotate at a desired rate.
In the head actuator 810, the head control circuit 811 receives sector identification signals from the host computer and feedback signals from the read/write head and generates control signals which are applied to highside MOSFETs 815 and lowside MOSFETs 816. The amplitudes of the control signals are determined by the sector identification signals and from the feedback signals received from the read/write head. During normal operation, the head-retract circuit 812 does not function.
When a battery failure occurs, V.sub.BAT drops to ground (or some other low potential), thereby reverse-biasing the Schottky diode 850 and isolating V.sub.CC from V.sub.BAT. In addition, the spindle driver 820 and head actuator 810 operate as follows.
In the spindle driver 820, kinetic energy stored in the spindle motor 830 generates a back emf at its input. The intrinsic anti-parallel diode A1 of each highside MOSFET 825 is forward-biased by the back emf and, with Schottky diode 850 reverse-biased, the power generated by motor 830 is delivered to head actuator 810. Ideally, the back emf generated by motor 830 lasts long enough to enable head actuator 810 to move the head to a safe sector of the disk.
In the head actuator 810, the loss of V.sub.BAT is sensed by the head-control circuit 811 and the head-retract circuit 812. The head-retract circuit 812 applies a retract control signal to the gates of the highside MOSFETs 815 and the lowside MOSFETs 816, which causes the voice-coil motor 840 to move the read/write head over a safe sector of the information-bearing disk.
A problem associated with prior art head storage device is that a voltage of 1.4 volts or more is required to drive the MOSFETs of the head actuator 810. This is the gate to source potential (Vgs) that is required to turn on MOSFETs 815 and 816. Most power MOSFETs, whether discrete or integrated, have little current drive capability when Vgs is equal to or less than 1.4 volts, and must be greatly oversized to satisfy this condition. During normal operation, V.sub.CC is typically 4.5 volts. Since the MOSFETs in the head actuator generally draw significant amounts of current, the loss of V.sub.BAT during a battery failure can cause V.sub.CC to drop from 4.5 volts to 1.4 volts very quickly.
Another problem with the prior art storage device is that the 0.5 volt diode drop associated with the Schottky diode 850 unnecessarily reduces the driving potential applied to the spindle motor 830, and hence reduces the back emf generated by the spindle motor 830 when the driving potential is removed. The back emf generated by motor 830 cannot be greater than V.sub.CC. In some cases, this reduction may cause V.sub.CC to decay before a successful emergency head-retract procedure can be performed.
Moreover, because a current of several amps or more is required to drive the spindle motor 830 and the voice-coil motor 840, up to a watt of power is lost due to the Schottky diode 850 during normal operation. For example, since a typical portable computer is powered by a 5 volt battery, the 0.5 volt diode drop due the Schottky diode 850 reduces the available motor driving potential by 10%. Further, with the trend toward portable computers driven by 3 volt batteries, this reduction in available driving potential increases to 17%.